Electron injection composition for light emitting element, light emitting element, and light emitting device

ABSTRACT

In the present invention, an electron injection composition for a light-emitting element, comprising a benzoxazole derivative represented by general formula 1 and at least one of an alkali metal, an alkali earth metal, and a transition metal, is used to form an electron injection layer in a portion of a layer including luminescent material in a light-emitting element, and it is an object of the present invention to provide, by using the composition, a light-emitting element that has more superior characteristics and a longer lifetime as compared to conventional ones. 
                         
wherein Ar represents an aryl group, and each of R 1  to R 4  represents hydrogen, halogen, a cyano group, an alkyl group having 1 to 10 carbon atoms, a haloalkyl group having 1 to 10 carbon atoms, an alkoxyl group having 1 to 10 carbon atoms, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electron injection composition for alight-emitting element, a light-emitting element formed with the use ofthe electron injection composition, and a light-emitting device that hasthe light-emitting element.

2. Description of the Related Art

A light-emitting element that uses a material as a light emitter, whichhas features such as a thin thickness and lightweight, a high speedresponse, and low DC voltage drive, has been expected to be applied to anext-generation flat panel display. In addition, it is said that alight-emitting device that has light-emitting elements arranged in amatrix shape is superior in having a wide view angle and a high level ofvisibility, as compared to a conventional liquid crystal display device.

A light-emitting element is said to have an emission mechanism that: anelectron injected from a cathode and a hole injected from an anode arerecombined in the luminescence center of a layer including luminescentmaterial to form an excited molecule when a voltage is applied with thelayer including the luminescent material between the pair of electrodes;and energy is released to emit light while the excited molecule movesback toward a ground state. As the excited state, a singlet excitedstate and a triplet excited state are known, and luminescence is said tobe possible through any of the singlet excited state and the tripletexcited state.

As for such a light-emitting element as this, there are a lot ofproblems depending on materials against improving characteristics of theelement, and therefore, in order to overcome the problems, the structureof the element has been improved and materials for the element has beendeveloped, for example.

One of the problems depending on materials is that there are fewappropriate materials for forming a transparent conductive film. As anappropriate material for forming a transparent conductive film, amaterial that has a large work function (specifically a work function of4.0 eV or more) such as ITO (indium tin oxide) or IZO (indium zincoxide) of indium oxide mixed with zinc oxide (ZnO) at 2 to 20% is known.Since the transparent conductive film as mentioned above is used for atransparent electrode to take light from a light-emitting element to theoutside, it is usually the case that the transparent electrode functionsas anode of the light-emitting element.

On the contrary, it is reported that an electron injection from atransparent electrode can be improved to make the transparent electrodeformed of a material that has a large work function such as ITO functionas a cathode when a layer for improving an electron injection from anelectrode (hereinafter, referred to as an electron injection layer) isformed in contact with the transparent electrode (refer to Non-PatentDocuments 1 and 2, for example). In this case, an electron transportmaterial (for example, tris(8-quinolinolato)aluminum (hereinafter,referred to as Alq₃), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline(also referred to as bathocuproin and hereinafter, referred to as BCP),and copper phthalocyanine (hereinafter, referred to as Cu-Pc)) is dopedwith an alkali metal to form the electron injection layer.

In addition, it is also reported that an electron injection from anelectrode can be remarkably improved in the case where BCP amongelectron injection layers, which is known as a material that is superiorin transporting only electron, is doped with Li.

However, it is difficult to keep amorphous when BCP is used to form afilm, and BCP also has a defect of being easy to crystallize with time.Therefore, in the case of forming an element, deterioration in devicecharacteristics such as fluctuations in luminance is caused due to achange in luminous efficiency, and the element also has a problem of ashortened lifetime due to deterioration in luminance.

(Non-Patent Document 1)

Junji Kido, Toshio Matsumoto, Applied Physics Letters, Vol. 73, No. 20(16 Nov. 1998), 2866-2868

(Non-Patent Document 2)

G. Parthasarathy, C. Adachi, P. E. Burrows, S. R. Forrest, AppliedPhysics Letters, Vol. 76, No. 15 (10 Apr. 2000), 2128-2130

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electroninjection composition for a light-emitting element, which is superior ininjecting electrons and is hard to crystallize with time in the case offorming a film to include the electron injection composition, and it isalso an object of the present invention to provide, by using theelectron injection composition, a light-emitting element that has moresuperior characteristics and a longer lifetime as compared toconventional ones, and a light-emitting device that uses thelight-emitting element.

The inventor has found that a benzoxazole derivative is a material thatis superior in transporting electrons and is hard to crystallize withtime in the case of forming a film to include the benzoxazolederivative, and that a layer that is superior in injecting electrons isformed when at least one of an alkali metal, an alkali earth metal, anda transition metal is included in the benzoxazole derivative. In thisconnection, the inventor has suggested that a benzoxazole derivative isused in a portion of a layer including a luminescent material in alight-emitting element that has the layer including the luminescentmaterial between a pair of electrodes.

In other words, the present invention provides an electron injectioncomposition for a light-emitting element, including a materialrepresented by general formula 1 and at least one of an alkali metal, analkali earth metal, and a transition metal, wherein a molar ratio of thematerial represented by general formula 1 to said one of the alkalimetal, the alkali earth metal, the transition metal is from 1:0.1 to1:10, more preferably from 1:0.5 to 1:2.

(where Ar represents an aryl group, and R₁ to R₄ represent individuallyhydrogen, halogen, a cyano group, an alkyl group having 1 to 10 carbonatoms, a haloalkyl group having 1 to 10 carbon atoms, an alkoxyl grouphaving 1 to 10 carbon atoms, a substituted or unsubstituted aryl group,or a substituted or unsubstituted heterocyclic group.)

Further, it is preferable that a ratio of an amount of substance of thematerial represented by general formula 1 to the total amount ofsubstance of the alkali metal, the alkali earth metal, and thetransition metal is from 1:0.1 to 1:10, more preferably from 1:0.5 to1:2.

Besides, the present invention also includes a light-emitting elementincluding an electron injection layer formed by using the electroninjection composition since the electron injection composition describedabove is superior in injecting electrons.

In other words, a light-emitting element comprising at least a layerincluding a luminescent material between a pair of electrodes, where thelayer including the luminescent material comprises a layer including amaterial represented by the general formula 1 and at least one of analkali metal, an alkali earth metal, and a transition metal.

In the light-emitting element, the layer includes the materialrepresented by the general formula 1 and at least one of the alkalimetal, the alkali earth metal, and the transition metal, wherein a molarratio of the material represented by general formula 1 to said one ofthe alkali metal, the alkali earth metal, the transition metal is from1:0.1 to 1:10, more preferably from 1:0.5 to 1:2.

In addition, in the light-emitting element, the layer including thematerial represented by the general formula 1 and at least one of thealkali metal, the alkali earth metal, and the transition metal is formedin contact with one of the pair of electrodes, where the electrode is anelectrode that functions as a cathode while the other is an electrodethat functions as an anode. In the case where the layer functions as anelectron injection layer in contact with the electrode that functions asa cathode, the electrode has a transmissivity of 70% or more to visiblelight.

The present invention includes the case that each of the pair ofelectrodes has a transmissivity of 70% or more to visible light.

Further, the present invention includes a light-emitting device that hasthe light-emitting element described above as a part.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram describing a light-emitting element according to thepresent invention;

FIG. 2 is a diagram describing a light-emitting element with aboth-emission structure;

FIG. 3 is a diagram describing a light-emitting element with atop-emission structure;

FIG. 4 is a diagram describing a light-emitting element with atop-emission structure;

FIGS. 5A and 5B are diagrams describing a light-emitting device;

FIGS. 6A to 6G are diagrams describing electronic devices; and

FIG. 7 is a graph showing a change in normalized luminance with timewith respect to each of a light-emitting element according to thepresent invention and a conventional light-emitting element.

DETAILED DESCRIPTION OF THE INVENTION

A light-emitting element according to the present invention has astructure shown in FIG. 1. Basically, a layer 102 including aluminescent material is sandwiched between a pair of electrode (a firstelectrode 101 and a second electrode 103), and the layer 102 includingthe luminescent material has at least a light-emitting layer 104 and anelectron injection layer 105 comprising an electron injectioncomposition for a light-emitting element according to the presentinvention. In addition, layers such as a hole injection layer, a holetransport layer, a hole blocking layer, and an electron transport layerare appropriately combined for forming the layer 102 including theluminescent material. Here, the case where the first electrode 101formed on a substrate 100 functions as an anode while the secondelectrode 103 functions as a cathode will be described.

In the present invention, the electron injection composition for alight-emitting element is formed by doping a benzoxazole derivative(BzOS) represented by the following general formula 1 with one of analkali metal, an alkali earth metal, and a transition metal, and theelectron injection layer 105 is provided in contact with the cathode ofthe light-emitting element.

(where Ar represents an aryl group, and R₁ to R₄ represent individuallyhydrogen, halogen, a cyano group, an alkyl group having 1 to 10 carbonatoms, a haloalkyl group having 1 to 10 carbon atoms, an alkoxyl grouphaving 1 to 10 carbon atoms, a substituted or unsubstituted aryl group,or a substituted or unsubstituted heterocyclic group.)

The benzoxazole derivative (BzOS) represented by the general formula 1includes specific materials shown by structure formulas 2 to 4.

4,4′-Bis(5-methyl benzoxazol-2-yl)stilbene

2-(4-Biphenylyl)-6-phenylbenzoxazole

2,5-Bis(5′-tert-butyl-2′-benzoxazolyl)thiophene

As the metal (the alkali metal, the alkali earth metal, or thetransition metal) included with the benzoxazole derivative, a metal suchas Li, Na, K, Rb, Cs, Fr, Mg, Ca, Sr, Ce, or Yb is specifically used,and the molar ratio of the benzoxazole derivative to the metal is from1:0.1 to 1:10, more preferably from 1:0.5 to 1:2. It is preferable thatthe electron injection layer 105 has a film thickness of 5 to 50 nm,more preferably, 10 to 30 nm.

As an anode material that is used for the first electrode 101, it ispreferable to use a metal, an alloy, an electrically conductivecompound, or a mixture thereof, which has a large work function (a workfunction of 4.0 eV or more). As a specific example of the anodematerial, a metal such as gold (Au), platinum (Pt), titanium (Ti),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), palladium (Pd), or a nitride of a metalmaterial (TiN) can be used in addition to ITO (indium tin oxide) and IZO(indium zinc oxide) of indium oxide mixed with zinc oxide (ZnO) at 2 to20%.

On the other hand, as a cathode material that is used for the secondelectrode 103, it is preferable to use a metal, an alloy, anelectrically conductive compound, and a mixture of these, which have asmall work function (a work function of 3.8 eV or less). As a specificexample of the cathode material, a transition metal that includes arare-earth metal can be used in addition to an element belonging toGroup 1 or Group 2 of the periodic table of the element (an alkali metalsuch as Li or Cs, or an alkali earth element such as Mg, Ca, or Sr), analloy (such as Mg:Ag or Al:Li) including the element, and a compound(such as LiF, CsF, or CaF₂) including the element, and it is alsopossible to further laminate a metal (including an alloy) such as Al,Ag, or ITO to form the second electrode 103.

Since the electron injection layer that is superior in injectingelectrons is provided in contact with the cathode in the presentinvention, it is also possible to use materials that have a large workfunction, such as ITO and IZO mentioned above.

A thin film of the anode material and a thin film of the cathodematerial are formed by a method such as evaporation or sputtering torespectively form the anode and the cathode, which preferably have afilm thickness of 5 to 500 nm.

In the light-emitting element according to the present invention, lightgenerated by recombination of carriers in the layer 102 including theluminescent material is emitted from one or both of the first electrode101 (anode) and the second electrode 103 (cathode) to the outside. Inother words, the first electrode 101 is formed of a transparent materialin the case where light is emitted from the first electrode 101 whilethe second electrode 103 is formed of a transparent material in the casewhere light is emitted from the second electrode 103. In the case wherelight is emitted from the both electrodes, each of the both electrodesis formed of a transparent material.

For the layer 102 including the luminescent material, known materialscan be used, and any of a low molecular weight material and a highmolecular weight can be used. The material for forming the layer 102including the luminescent material includes not only organic compoundsbut also an inorganic compound included in a portion of the layer 102including the luminescent material.

The following will describe specific materials that are used for a holeinjection layer, a hole transport layer, a light-emitting layer, or anelectron transport layer for forming the layer 102 including theluminescent material.

As a hole injection material for forming a hole injection layer, aporphyrin-based compound is efficient among organic compounds. Forexample, phthalocyanine (hereinafter, referred to as H₂-Pc) and copperphthalocyanine (hereinafter, referred to as Cu-Pc) can be used. Inaddition, a material of a conductive polymer compound subjected tochemical doping such as polyethylene dioxythiophene (hereinafter,referred to as PEDOT) doped with polystyrene sulfonate (hereinafter,referred to as PSS), can be used.

As a hole transport material for forming a hole transport layer, anaromatic amine-based compound (that is, one that has a bond of benzenering-nitrogen) is preferably used. As materials that are used widely,for example, in addition toN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(hereinafter, referred to as TPD), a derivative thereof such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (hereinafter, referredto as α-NPD) and star burst aromatic amine compounds such as4,4′,4″-tris(N-carbazolyl)-triphenylamine (hereinafter, referred to asTCTA), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (hereinafter,referred to as TDATA), and4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(hereinafter, referred to as MTDATA) are included.

As a light-emitting (luminescent) material for forming a light-emittinglayer, various fluorescent materials are efficient specifically inaddition to metal complexes such as tris(8-quinolinolato)aluminum(hereinafter, referred to as Alq₃),tris(4-methyl-8-quinolinolato)aluminum (hereinafter, referred to asAlmq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium (herenafter,referred to as BeBq₂),bis(2-methyl-8-quinolinolato)-(4-hydroxy-biphenylyl)-aluminum(abbreviation: BAlq), bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc(hereinafter, referred to as Zn(BOX)₂), andbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (hereinafter, referred toas Zn(BTZ)₂).

In the case of forming the light-emitting layer in combination with aguest material, triplet luminescent materials (phosphorescent materials)such as tris (2-phenylpyridine)iridium (hereinafter, referred to asIr(ppy)₃) and 2, 3, 7, 8, 12, 13, 17, 18-octaethyl-21H,23H-porphyrin-platinum (hereinafter, referred to as PtOEP) can be usedas the guest material in addition to quinacridone, diethyl quinacridone(DEQD), dimethyl quinacridone (DMQD), rubrene, perylene, cuomarin,cuomarin 545T (C545T), DPT, Co-6, PMDFB, BTX, ABTX, DCM, and DCJT.

As an electron transport material forming an electron transport layer, ametal complex that has a quinoline skeleton or a benzoquinolineskeleton, such as Alq₃, Almq₃, or BeBq₂, and BAlq that is a mixed ligandcomplex are preferred. In addition, a metal complex that has anoxazole-based ligand or a thiazole-based ligand, such as Zn(BOX)₂ andZn(BTZ)₂, can also be used. In addition to the metal complexes,oxadiazole derivatives such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (hereinafter,referred to as PBD) and1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(hereinafter, referred to as OXD-7), triazole derivatives such as3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1, 2, 4-triazole(hereinafter, referred to as TAZ) and3-(4-tert-butylphenyl)-4-4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(hereinafter, referred to as p-EtTAZ) can be used.

As described above, the light-emitting element according to the presentinvention can be obtained, where a portion of the layer 102 includingthe luminescent material includes the electron injection layer 105comprising the electron injection composition according to the presentinvention and the electron injection layer 105 is formed in contact withthe cathode (the second electrode 103 in the present embodiment mode).

Hereinafter, embodiments of the present invention will be described.

Embodiment 1

In the present embodiment, a light-emitting element according to thepresent invention in the case of a structure in which light generated ina layer including a luminescent material is emitted from both side offirst and second electrodes of a light-emitting element (hereinafter,referred to as a both-emission structure), which has an electroninjection layer provided in a portion of the layer including theluminescent material, will be described with reference to FIG. 2.

First, a first electrode 201 of a light-emitting element is formed on asubstrate 200. In the present embodiment, the first electrode 201functions as an anode. ITO that is a transparent conductive film is usedas a material to form the first electrode 201 with a film thickness of110 nm by sputtering.

Next, a layer 202 including a luminescent material is formed on thefirst electrode (anode) 201. The layer 202 including the luminescentmaterial in the present embodiment has a laminated structure comprisinga hole injection layer 211, a hole transport layer 212, a light-emittinglayer 213, an electron transport layer 214, and an electron injectionlayer 215.

The substrate that has the first electrode 201 formed is fixed in asubstrate holder of a commercially produced vacuum deposition systemwith a surface of the first electrode 201 down, Cu-Pc is put in anevaporation source equipped inside the vacuum deposition system, andthen the hole injection layer 211 is formed by evaporation withresistance heating to have a film thickness of 20 nm. As a material forforming the hole injection layer 211, a known hole injection materialcan be used.

Next, a material that is superior in transporting holes is used to formthe hole transport layer 212. It is possible to use a known holetransport material as a material for forming the hole transport layer212. In the present embodiment, α-NPD is deposited in the same way tohave a film thickness of 40 nm.

Next, the light-emitting layer 213 is formed, where a hole and anelectron are recombined to generate luminescence. In the presentembodiment, Alq₃ that serves as a host material and cuomarin 545T(C545T) that serves as a guest material are used as materials forforming the light-emitting layer 213 to form the light-emitting layer213 with a film thickness of 40 nm by co-evaporation so as to includeC545T at 1 wt %.

Next, the electron transport layer 214 is formed. As a material forforming the electron transport layer 214, a known electron transportmaterial can be used. In the present embodiment, Alq₃ is used to formthe electron transport layer 214 with a film thickness of 20 nm byevaporation.

Next, the electron injection layer 215 is formed. An electron injectioncomposition according to the present invention is used for the electroninjection layer 215. The electron injection composition includes abenzoxazole derivative represented by general formula 1 and one of analkali metal, an alkali earth metal, and a transition metal, wherein amolar ratio of the material represented by general formula 1 to said oneof the alkali metal, the alkali earth metal, the transition metal isfrom 1:0.1 to 1:10. In the present embodiment, the molar ratio of thebenzoxazole derivative represented by the structure formula 2 to Li thatis the alkali metal is 1:2 to form the electron injection 215 with afilm thickness of 20 nm by co-evaporation.

4,4′-Bis(5-methyl benzoxazol-2-yl)stilbene

After the hole injection layer 211, the hole transport layer 212, thelight-emitting layer 213, the electron transport layer 214, and theelectron injection later 215 are laminated to form the layer 202including the luminescent material in this way, the second electrode 203that functions as a cathode is formed by sputtering or evaporation.

Since the second electrode 203 is formed in contact with the electroninjection layer 215 that is superior in injecting electrons, ITO (110nm) is formed on the layer 202 including the luminescent material bysputtering in the present embodiment to obtain the second electrode 203.

As described above, the light-emitting element with a both-emissionstructure is formed.

Since the benzoxazole derivative included in the electron injectioncomposition for a light-emitting element, which is used for the electroninjection layer of the light-emitting element described in the presentembodiment, is a material that is superior in injecting electrons and ishard to crystallize in the case of forming a film to include thebenzoxazole derivative, a light-emitting element that has superiorcharacteristics and a long lifetime can be formed.

COMPARATIVE EXAMPLE 1

In order to be compared to the light-emitting element described inEmbodiment 1, a conventional light-emitting element that uses BCP for anelectron injection layer is manufactured.

In the same way as the light-emitting element described in Embodiment 1,layers up to an electron transport layer are formed. Then, the molarratio of BCP to Li that is the alkali metal is 1:2 to form an electroninjection layer with a film thickness of 20 nm by co-evaporation. Afterthat, a second electrode that functions as a cathode is formed on theelectron injection layer by sputtering or evaporation. In the presentembodiment, ITO (110 nm) is formed by sputtering.

FIG. 7 shows a graph showing a change in normalized luminance with timewith respect to each of the light-emitting element “Comparative Example1” manufactured in the present comparative example and thelight-emitting element “Embodiment 1” manufactured in Embodiment 1. Thechange in normalized luminance with time is a change with time inluminance normalized in accordance with an initial luminance set at100%, which is measured with a constant current density.

As is clear from FIG. 7, the light-emitting element of “Embodiment 1”has a luminance less lowering with time than the light-emitting elementof “Comparative Example 1”. In other words, the light-emitting elementof “Embodiment 1” has a longer lifetime. More specifically, a lifetimefor the normalized luminance with respect to the light-emitting elementof “Comparative Example 1” to reach 58% is 560 hours, while a lifetimefor the normalized luminance with respect to the light-emitting elementof “Embodiment 1” to reach 60% is 1800 hours. Accordingly, it isapparent that a light-emitting element that has more superiorcharacteristics and a longer lifetime can be formed by forming anelectron injection layer with the use of an electron injectioncomposition for a light-emitting element according to the presentinvention, which includes a benzoxazole derivative and at least one ofan alkali metal, an alkali earth metal, and a transition metal.

Embodiment 2

In the present embodiment, a specific structure in which light isemitted from a second electrode to be formed a layer including aluminescent material (hereinafter, referred to as a top-emissionstructure) in the case of a structure where light generated in a layerincluding a luminescent material is emitted from one side of alight-emitting element will be described with reference to FIG. 3.

Only different portion from the structure in Embodiment 1 will bedescribed here, and descriptions of component parts except a firstelectrode, which are the same as those of Embodiment 1, will be omitted.

A first electrode 301 in the present embodiment functions as an anode,from which no light generated in a layer 302 including a luminescentmaterial is emitted. In other words, a nitride or carbide of an elementbelonging to one of Group 4, Group 5, and Group 6 of the periodic tableof the elements, which has a large work function and shields light, suchas titanium nitride, zirconium nitride, titanium carbide, zirconiumcarbide, tantalum nitride, tantalum carbide, molybdenum nitride, andmolybdenum carbide, can be used as a material for forming the firstelectrode 301. In addition, a transparent conductive film that has alarge work function and a conductive film (a reflective film) that isreflective (including also the case of shielding light) can be laminatedto form the first electrode 301.

The present embodiment will describe the case where a reflectiveconductive film and a transparent conductive film that has a large workfunction are laminated to form the first electrode 301.

Specifically, a reflective conductive film 304 is formed by laminating aTi film to be a film thickness of 100 nm after forming an Al—Si film ona substrate 300 to be a film thickness of 300 nm, and ITO is formedthereon to be 20 nm as a transparent conductive film 305.

After forming the first electrode 301, a hole injection layer 311, ahole transport layer 312, a light-emitting layer 313, an electrontransport layer 314, an electron injection layer 315, and a secondelectrode 303 are sequentially laminated in the same way as Embodiment1.

In this way, the light-emitting element that has the top-emissionstructure in which light is emitted from only the second electrode canbe formed.

As mentioned in Embodiment 1, since the benzoxazole derivative includedin the electron injection composition for a light-emitting element,which is used for the electron injection layer of the light-emittingelement also in the case of the present embodiment, is a material thatis superior in injecting electrons and is hard to crystallize in thecase of forming a film to include the benzoxazole derivative, alight-emitting element that has superior characteristics and a longlifetime can be formed.

Embodiment 3

In the present invention, a structure in which light is emitted from aside of an electron injection layer 515 included in a layer 502including a luminescent material as shown in FIG. 4 (hereinafter,referred to as a top-emission structure) can also be employed in thecase of a structure where light generated in a layer including aluminescent material is emitted from one side of a light-emittingelement as described in Embodiment 2.

In the same way as Embodiment 2, as a first electrode 501, a reflectiveconductive film 504 is formed on a substrate 500 and then, a transparentconductive film 505 is formed thereon. After forming the first electrode501, a hole injection layer 511, a hole transport layer 512, alight-emitting layer 513, an electron transport layer 514, the electroninjection layer 515, and a second electrode 503 are sequentiallylaminated in the same way as Embodiments 1 and 2.

In the case of the present embodiment, since the second electrode(cathode) 503 that functions as an auxiliary electrode on a portion ofthe layer 502 including the luminescent material, light generated in thelayer 502 including the luminescent material is emitted from the otherportion on which the second electrode 503 is not formed. In the presentembodiment, it is not always necessary to use a transparent material asa material that is used for the second electrode 503 as long as acathode material that has a small work function, a material that shieldslight may be used.

Embodiment 4

In the present embodiment, a light-emitting device that has alight-emitting element according to the present invention in a pixelportion will be described with reference to FIGS. 5A and 5B. FIG. 5A isa top view showing the light-emitting device and FIG. 5B is a sectionalview of FIG. 5A cut along A-A′. Reference numeral 401 indicated by adotted line denotes a driver circuit portion (a source side drivercircuit), 402 is a pixel portion, and 403 is a driver circuit portion (agate side driver circuit). In addition, reference numerals 404 and 405denote a sealing substrate and a sealing agent, respectively. The insidesurrounded by the sealing agent 405 is a space 407.

A wiring 408 is provided for transmitting signals to be input to thesource side driver circuit 401 and the gate side driver circuit 403, andreceives signals such as a video signal, a clock signal, a start signal,and a reset signal, from FPC (Flexible Printed Circuit) 409 that servesas an external input terminal. Though only the FPC is shown in thefigure here, a printed wiring board (PWB) may be attached to the FPC.The light-emitting device in the specification includes not only alight-emitting device body but also a state where an FPC or a PWB isattached thereto.

Next, the sectional structure will be explained with reference to FIG.5B. The driver circuits and the pixel portion are formed over a devicesubstrate 410. Here, the source side driver circuit 401 as the drivercircuit portion and the pixel portion 402 are shown.

In the source side driver circuit 401, a CMOS circuit is formed of acombination of an n-channel TFT 423 and a p-channel TFT 424. The TFTsforming the driver circuit may be formed of a known CMOS circuit, PMOScircuit, or NMOS circuit. Although the present embodiment shows a driverintegrated type in which a driver circuit is formed over a substrate,which is not always necessary, the driver circuit can be formed not overthe substrate but at the outside thereof.

The pixel portion 402 is formed of a plurality of pixels, each includinga switching TFT 411, a current controlling TFT 412, and a firstelectrode 413 connected to a drain of the controlling TFT 412electrically. In addition, an insulator 414 is formed to cover an edgeof the first electrode 413. Here, a positive photosensitive acrylicresin film is used to form the insulator 414.

On the first electrode 413, a layer 416 including a luminescent materialand a second electrode 417 are respectively formed. Here, as a materialthat is used for the first electrode 413 functioning as an anode, it ispreferable to use a material that has a large work function. Forexample, in addition to single layers such as an ITO (indium tin oxide)film, an indium zinc oxide (IZO) film, a titanium nitride film, achromium film, a tungsten film, a Zn film, and a Pt film, structuressuch as a laminate of a titanium nitride film and a film includingaluminum as its main component and a three-layer structure of a titaniumnitride film, a film including aluminum as its main component, and atitanium nitride film, can be used. When a laminated structure isemployed, the wiring has a low resistance, favorable ohmic contact canbe taken, and it is possible to function as an anode.

The layer 416 including the luminescent material is formed byevaporation that uses an evaporation mask or by inkjet. The layer 416including the luminescent material includes an electron injection layercomprising an electron injection composition for a light-emittingelement according to the present invention. In addition, alight-emitting layer, a hole injection layer, a hole transport layer,and an electron transport layer are included in the layer 416 includingthe luminescent material. In forming these layers, a low molecularweight material, a middle molecular weight material (including anoligomer and a dendrimer) and a high molecular weight material can beused. In addition, it is often the case that an organic compound is usedfor a single layer or a laminate in the case of forming the layerincluding the luminescent material. However, the present inventionincludes a structure in which an inorganic compound is used for a partof a film comprising an organic compound.

As a material that is used for the second electrode (cathode) 417 formedon the layer 416 including the luminescent material, ITO that is atransparent conductive film is used.

Further, the sealing substrate 404 and the substrate 410 are bonded withthe sealing agent 405 to obtain a structure where a light-emittingelement 418 is equipped in the space 407 surrounded by the substrate410, the sealing substrate 404, and the sealing agent 405. The space 407also includes a structure of filling with the sealing agent 405 inaddition to a case of filling with inert gas (such as nitrogen orargon).

It is preferable to use epoxy resin for the sealing agent 405. Inaddition, it is desirable to use a material that allows no permeation ofmoisture or oxygen to as much as possible. Further, as a material thatis used for the sealing substrate 404, a plastic substrate comprisingFRP (Fiberglass-Reinforced Plastics), PVF (polyvinylfluoride), Mylar,polyester, or acrylic can be used besides a glass substrate and a quartssubstrate.

In this way, the light-emitting device that has the light-emittingelement formed according to the present invention can be obtained.

The light-emitting device shown in the present embodiment can be usedfreely in combination with any of the structures of the light-emittingelements shown in Embodiments 1 to 3.

Embodiment 5

In the present embodiment, various electronic devices completedincluding a light-emitting device, for example formed according toEmbodiment 4 will be described.

As electronic devices manufactured with the use of a light-emittingdevice according to the present invention, devices such as a videocamera, a digital camera, a goggle-type display (head mount display), anavigation system, a sound reproduction device (such as an in-car audiosystem or an audio set), a laptop personal computer, a game machine, apersonal digital assistance (such as a mobile computer, a cellularphone, a portable game machine, or an electronic book), an imagereproduction device equipped with a recording medium (specifically, adevice equipped with a display device that can reproduce a recordingmedium such as a digital versatile disc (DVD) and display the image) canbe given. FIGS. 6A to 6G show specific examples of such electronicdevices.

FIG. 6A is a display device, which includes a frame body 2001, a support2002, a display portion 2003, a speaker portion 2004, and a video inputterminal 2005. A light-emitting device formed according to the presentinvention is used for the display portion 2003 to manufacture thedisplay device. The display device includes all devices for displayinginformation such as for a personal computer, for receiving TV broadcasting, and for displaying an advertisement.

FIG. 6B is a laptop personal computer, which includes a main body 2201,a frame body 2202, a display portion 2203, a keyboard 2204, an externalconnection port 2205, and a pointing mouse 2206. A light-emitting deviceformed according to the present invention is used for the displayportion 2203 to manufacture the laptop personal computer.

FIG. 6C is a mobile computer, which includes a main body 2301, a displayportion 2302, a switch 2303, an operation key 2304, and an infrared port2305. A light-emitting device formed according to the present inventionis used for the display portion 2302 to manufacture the mobile computer.

FIG. 6D is a portable image reproduction device equipped with arecording medium (specifically, a DVD reproduction device), whichincludes a main body 2401, a frame body 2402, a display portion A 2403,a display portion B 2404, a recording medium (such as DVD) readingportion 2405, an operation key 2406, and a speaker portion 2407. Thedisplay portion A 2403 is used mainly for displaying image informationwhile the display portion B 2404 is used mainly for displaying characterinformation, and a light-emitting device formed according to the presentinvention is used for these display portion A 2403 and display portion B2404 to manufacture the portable image reproduction device equipped withthe recording medium. The image reproduction device equipped with therecording medium further includes a home game machine.

FIG. 6E is a goggle-type display (head mount display), which includes amain body 2501, a display portion 2502, and an arm portion 2503. Alight-emitting device formed according to the present invention is usedfor the display portion 2502 to manufacture the goggle-type display.

FIG. 6F is a video camera, which includes a main body 2601, a displayportion 2602, a frame body 2603, an external connection port 2604, aremote control receiving portion 2605, an image receiving portion 2606,a battery 2607, a sound input portion 2608, an operation key 2609, andan eye piece 2610. A light-emitting device formed according to thepresent invention is used for the display portion 2602 to manufacturethe video camera.

FIG. 6G is a cellular phone, which includes a main body 2701, a framebody 2702, a display portion 2703, a voice input portion 2704, a voiceoutput portion 2705, an operation key 2706, an external connection port2707, and an antenna 2708. A light-emitting device formed according tothe present invention is used for the display portion 2703 tomanufacture the cellular phone.

As described above, a light-emitting device formed according to thepresent invention is quite widely applied. In addition, since alight-emitting element included in the light-emitting device has anelectron injection layer for enhancing an electron injection from anelectrode in a portion of a layer including a luminescent material and amaterial that is hard to crystallize is used to form the electroninjection layer, the light-emitting element has a low driving voltageand a long lifetime. Therefore, it is possible to reduce powerconsumption and extend a lifetime by applying this light-emitting deviceto electronic devices in all fields.

In the present invention, an electron injection from an electrode thatfunctions as a cathode can be enhanced by forming an electron injectionlayer with the use of an electron injection composition for alight-emitting element, which includes a benzoxazole derivative and atleast one of an alkali metal, an alkali earth metal, and a transitionmetal. Further, since the benzoxazole derivative is hard to crystallizein the case of forming a film to include the benzoxazole derivative, itis possible to provide a light-emitting element that has more superiorcharacteristics and a longer lifetime as compared to conventional ones,and a light-emitting device that uses the light-emitting element.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Such changes and modifications should beconstructed as being included in the scope of the attached claims.

1. A light emitting element comprising: a first electrode; a secondelectrode comprising a transparent material; and an electron injectionlayer and a light emitting layer, formed between the the first electrodeand the second electrode, wherein the electron injection layer is incontact with the second electrode, wherein the electron injection layercomprises: one of materials represented by structure formula 2 to 4; andat least one of an alkali metal, an alkali earth metal, and a transitionmetal, wherein a molar ratio of one of the materials represented bystructure formula 2 to 4 to said one of the alkali metal, the alkaliearth metal, and the transition metal is from 1:0.1 to 1:10,


2. The light emitting element according to claim 1, wherein a molarratio of one of the materials represented by structure formula 2 to 4 tosaid one of the alkali metal, the alkali earth metal, and the transitionmetal is from 1:0.5. to 1:2.
 3. A light emitting element comprising: afirst electrode; a second electrode comprising a transparent material;and an electron injection layer and a light emitting layer, formedbetween the first electrode and the second electrode, wherein theelectron injection layer is in contact with the second electrode,wherein the electron injection layer comprises: one of materialsrepresented by structure formula 2 to 4; and at least one of an alkalimetal, an alkali earth metal, and a transition metal, wherein a ratio ofan amount of substance of one of the materials represented by structureformula 2 to 4 to the total amount of substance of the alkali metal, thealkali earth metal, and the transition metal is from 1:0.1 to 1:10,


4. The light emitting element according to claim 3, wherein a ratio ofthe amount of substance of one of the materials represented by structureformula 2 to 4 to the total amount of substance of the alkali metal, thealkali earth metal, and the transition metal is from 1:0.5 to 1:2.
 5. Alight-emitting element comprising: a first electrode; a second electrodecomprising a transparent material; and at least a layer including aluminescent material between the first Electrode and the secondelectrode, wherein the layer is in contact with the second electrode,wherein the layer comprises a layer including one of materialsrepresented by structure formula 2 to 4 and at least one of an alkalimetal, an alkali earth metal, and a transition metal,


6. The light-emitting element according to claim 5, wherein a molarratio of one of the materials represented by structure formula 2 to 4 tosaid one of the alkali metal, the alkali earth metal, and the transitionmetal is from 1:0.1 to 1:10.
 7. The light-emitting element according toclaim 5, wherein a molar ratio of one of the materials represented bystructure formula 2 to 4 to said one of the alkali metal, the alkaliearth metal, and the transition metal is from 1:0.5 to 1:2.
 8. Thelight-emitting element according to claim 5, wherein the layer includingone of the materials represented by structure formula 2 to 4 and atleast one of the alkali metal, the alkali earth metal, and thetransition metal is formed in contact with one of the pair of electrodeswhich functions as a cathode.
 9. The light-emitting element according toclaim 5, wherein one of the pair of electrodes has a transmissivity of70% or more to visible light.
 10. The light-emitting element accordingto claim 5, wherein each of the pair of electrodes has a transmissivityof 70% or more to visible light.
 11. An electronic device comprising thelight-emitting element according to claim 5, wherein the electronicdevice is selected from the group consisting of a video camera, adigital camera, a goggle-type display, a navigation system, a soundreproduction device, a laptop personal computer, a game machine, apersonal digital assistant, a cellular phone, a portable game machine,an electronic book, and an image reproduction device.